1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the device, particularly to technique improving operation sustaining voltage characteristic of high sustaining voltage MOS transistor for high voltage of power source (HV-VDD) used for an LCD driver, an EL driver and so on.
2. Description of the related Art
A semiconductor device according to the related art will be described below referring a section view of an LDD type high sustaining voltage MOS transistor shown in FIG. 10.
In FIG. 10, a gate electrode 53 is formed on a P type semiconductor substrate (P-Sub) 51 through a gate insulation film 52. An N+ type source region 54 is formed so as to be adjacent to one end of said gate electrode 53, an Nxe2x88x92 type drain region 56 is formed facing said source region 54 through a channel region and further separated from the other end of the gate electrode 53, and an N+ type drain region 57 is formed so as to be included in an Nxe2x88x92 type drain region 56.
In the prior art, a low concentration Nxe2x88x92 type drain region 56 is formed by thermal diffusion of about 1000xc2x0 C. to 1100xc2x0 C. so as to form a gentle slope and a deep diffusion layer.
However, even with such the construction, voltage between source and drain (BVDS: sustaining voltage at OFF) is high, but sustaining voltage (VSUS: sustaining voltage at ON) being operation sustaining voltage of the voltage is about 30 V at most in the prior art.
A mechanism decreasing the above-mentioned operation sustaining voltage will be described below.
In such the N channel type high sustaining voltage MOS transistor, a horizontal bipolar transistor 60 having the drain region 57 as corrector (N+), the source region 54 as emitter (N+), and the semiconductor substrate 51 as base (P) is formed parasitically as shown in FIG. 11 and FIG. 12. Decreasing of operation sustaining voltage VSUS even if voltage between source and drain BVDS being sustaining voltage at OFF is high is caused by ON of the parasitical bipolar transistor 60. Thus, operation range of the N channel type high sustaining voltage MOS transistor is limited and operation at all over the range is difficult.
An operation of said bipolar transistor 60 will be described below.
AS shown in FIG. 11, gate voltage (VG)( greater than Vt: threshold voltage) is added to the gate electrode 53, voltage of a drain electrode (VD)( greater than  greater than VG) contacting the drain region 57 is added, and a positive feedback loop described later (refer FIG. 12) is formed in the case of ON of the MOS transistor.
That is, {circle around (1)} avalanche multiplication generates in a depletion layer by electron of a channel region 62 accelerated at a depletion layer near the drain region 57 so as to generate a pair of an electron and a hole. {circle around (2)} Said hole flows in the substrate (substrate current: ISub) {circle around (3)} Said substrate current (ISub) generates voltage slope in the semiconductor substrate 51 to raise substrate voltage. {circle around (4)} Junction between the source region 54 and the substrate 51 is biased to forward direction. {circle around (5)} Electron is implanted from the source region 54 to the substrate 51. {circle around (6)} The implanted electron reaches the drain region 57 and further occurs avalanche multiplication.
Thus, by forming the positive feedback of {circle around (1)} to {circle around (6)}, large current flows in the device so as to break the device.
Therefore, in design of the N channel type high sustaining voltage MOS transistor, conditions of the design is set considering the above-mentioned phenomenon. First, a transistor construction decreasing substrate current (ISub) is adopted because operation sustaining voltage (VSUS) becomes small as substrate current (ISub) becomes large, and second, the conditions are decided so as to decrease substrate current (ISub) at an actually used region.
FIG. 4 is a substrate current (ISub) vs. gate voltage (VG) characteristic view, in the figure, double humps characteristic of substrate current (ISub) at high region in gate voltage (VG) rises about the conventional N channel type high sustaining voltage MOS transistor (shown with a dotted line in the figure). Therefore, operation sustaining voltage (VSUS) is low shown in drain current (ID) vs. drain voltage (VD) characteristic view of FIG. 5 and a characteristic view showing operation sustaining voltage of FIG. 6.
The double humps characteristic is caused by concentration of electric field by spreading the depletion layer near the N+ drain region at high region in gate voltage (VG).
Although it is considered to increase ion implantation volume(doze) and to rise concentration of Nxe2x88x92 type drain region as shown in FIG. 6 to improve operation sustaining voltage (VSUS), the conventional semiconductor device is not improved enough in sustaining voltage as shown with white circles. Because concentration of end portion A of the Nxe2x88x92 type drain region 56 shown in FIG. 12 rises conversely, problems of increase of short channel effect by that depletion layer spreads to the channel region 55 direction, increase of snap back phenomenon by increase of peak value of substrate current (ISub), and further decrease of voltage between source and drain (BVDS) occur. Therefore, there is not effective means improving operation-sustaining voltage.
Therefore, an object of the invention is to provide a semiconductor device capable of improving operation sustaining voltage and a method of manufacturing the device.
A semiconductor device of the invention has a gate electrode formed on at least one conductive type semiconductor substrate through a gate insulation film, a low concentration reverse conductive type drain region formed so as to be adjacent to the gate electrode, a high concentration reverse conductive type drain region separated from the other end of said gate electrode and included in said low concentration reverse conductive type drain region, and a middle concentration reverse conductive type layer having a lower impurity concentration near the gate electrode than said high concentration reverse conductive type drain region, said middle concentration reverse conductive type layer is formed at a region spanning at least from said gate electrode to said high concentration reverse conductive type drain region. Therefore, the semiconductor device is characterized in improving operation-sustaining voltage.
Preferably said middle concentration reverse conductive type layer is formed at a region spanning at least from said gate electrode to said high concentration reverse conductive type drain region so that the impurity concentration gradually becomes high from said gate electrode to said high concentration reverse conductive type drain region or said high impurity concentration reverse conductive type source and drain region.
Preferably said middle concentration reverse conductive type layer is formed at a region spanning at least from said gate electrode to said high concentration reverse conductive type drain region so that the impurity concentration becomes high step by step from said gate electrode to said high concentration reverse conductive type drain region or said high impurity concentration reverse conductive type source and drain region.
A method of manufacturing a semiconductor device has processes of forming a low concentration reverse conductive type drain region on at least one conductive type semiconductor substrate, forming a gate insulation film at the entire surface of said semiconductor substrate, forming a gate electrode overlapping at least upper side of said drain region by patterning after forming a conductive film on the entire surface, forming a high concentration reverse conductive type source region adjacent to one end of said gate electrode and a high concentration reverse conductive type drain region separated from the other end of said gate electrode and included in said low concentration reverse conductive type drain region, and forming a middle concentration reverse conductive type layer formed at a region spanning from said gate electrode to said high concentration reverse conductive type drain region or said high impurity concentration reverse conductive type source and drain region so that impurity concentration becomes low near gate electrode.(lower than said high impurity concentration reverse conductive type source and drain region)
Said middle concentration reverse conductive type layer is formed at a region spanning from the gate electrode to said high concentration reverse conductive type drain region or said high impurity concentration reverse conductive type source and drain region by ion implantation from oblique upper side by using a photoresist covering the gate electrode as a mask at said forming process of the middle concentration reverse conductive type layer.
Said middle concentration reverse conductive type layer is formed at a region spanning from the gate electrode to said high concentration reverse conductive type drain region or said high impurity concentration reverse conductive type source and drain region by ion implantation so as to penetrate a side wall insulating film formed on a side wall of the gate electrode, having a tapered shape at said forming process of the middle concentration reverse conductive type layer.